ABSTRACT Cancer cells require a steady stream of nutrients to support their oncogene-driven growth. However, the blood vessels that supply these nutrients are often tortuous and leaky. Desmoplasia can also lead to elevated interstitial pressure that collapses tumor blood vessels, further compromising nutrient delivery. Cancer cells overcome these supply-side limitations by scavenging macromolecules from the microenvironment. One scavenging strategy employed by tumors is macropinocytosis, a process by which extracellular material is non- specifically engulfed and then degraded in the lysosome to produce nutrients. Oncogenic mutations in RAS, activation of the PI3K pathway, and EGFR and WNT signaling drive macropinocytosis in pancreatic, prostate, lung, colon, bladder, and breast cancer cell lines. When provided with macropinocytic fuel, these cancer cells are able to proliferate in nutrient-limiting conditions. However, is not currently clear whether the quantity and quality of material present in the tumor microenvironment is sufficient for macropinocytosis to make a significant contribution to tumor anabolism. All published studies have depended upon EIPA for in vivo macropinocytosis inhibition. EIPA is an inhibitor of Na+/H+ exchangers that has pleiotropic anti-neoplastic effects independent of macropinocytosis inhibition. What is currently lacking is a strategy to selectively disrupt macropinocytosis in vivo. As a result, it has not been possible to accurately define the contribution of macropinocytosis to tumor growth or the potential therapeutic value of targeting this pathway. Given that nutrient recycling via autophagy plays a major role in both tumor progression and therapeutic resistance, it is likely that nutrient scavenging through macropinocytosis will play a similarly important role. Aim 1 of this proposal will assess the extent to which selective macropinocytosis inhibition limits tumor growth. Aim 2 will evaluate the role of macropinocytosis in therapeutic resistance. Aim 3 will dissect the signals that promote macropinosome formation in tumor cells. Completing these studies will fill major gaps in our knowledge and could lead to new single-agent and/or combination therapies for cancer. Because some of the most difficult to treat cancers are macropinocytic (e.g. pancreas, KRAS+ colorectal, triple-negative breast, and castration- resistant prostate cancers), these studies have the potential to make a significant impact on patient survival.